The present invention relates to a cooling apparatus, and more particularly to a cooling apparatus for cooling an optical element in an exposure apparatus that exposes an object, such as a single crystal substrate and a glass plate for a liquid crystal display (“LCD”). The present invention is suitable, for example, for an exposure apparatus that uses ultraviolet (“UV”) and extreme ultraviolet (“EUV”) light as an exposure light source.
Reduction projection exposure apparatus have been conventionally employed which use a projection optical system to project a circuit pattern formed on a mask or a reticle onto a wafer, etc. to transfer the circuit pattern, in manufacturing such fine semiconductor devices as semiconductor memories and logic circuits in photolithography technology.
The minimum critical dimension (“CD”) to be transferred by the projection exposure apparatus or resolution is proportionate to a wavelength of light used for exposure, and inversely proportionate to the numerical aperture (“NA”) of the projection optical system. The shorter the wavelength is, the better the resolution is. Recent demands for finer semiconductor devices have promoted a shorter wavelength of ultraviolet light from an ultra-high pressure mercury lamp (i-line with a wavelength of approximately 365 nm) to KrF excimer laser (with a wavelength of approximately 248 nm) and ArF excimer laser (with a wavelength of approximately 193 nm).
However, the lithography using the ultraviolet light has the limit to satisfy the rapidly progressing fine processing of semiconductor devices, and a reduction projection optical system using EUV light with a wavelength of 10 to 15 nm shorter than that of the ultraviolet (referred to as an “EUV exposure apparatus” hereinafter) has been developed for efficient transfers of very fine circuit patterns.
Due to the remarkably increasing light absorption in a material with the shorter wavelength of the exposure light, it is difficult to use refraction elements or lenses for visible light and ultraviolet light. No glass material is viable to a wavelength range of the EUV light, and a reflection-type or cataoptric optical system uses only a reflective element or mirror, such as a multilayer mirror.
A mirror does not reflect all the exposure light, but absorbs the exposure light of 30% or greater. The absorbed exposure light causes residual heat, deforms a surface shape of the mirror, and deteriorates its optical performance, in particular, imaging performance. Thus, the mirror is made of a low thermal expansion glass, for example, having a coefficient of linear expansion of 10 ppb, so as to reduce a mirror's shape change as the temperature changes.
Since the EUV exposure apparatus is used to expose circuit patterns of 0.1 μm or smaller and required to meet very high critical dimension accuracy, only a deformation of about 0.1 nm or smaller is permissible on the mirror surface. When the mirror has a coefficient of linear expansion of 10 ppb, the mirror surface deforms as the temperature rises and the mirror surface shape changes by 0.1 nm when the temperature rises by 0.2° C.
Accordingly, as shown in FIG. 14, the instant assignee has already proposed to arrange a radiation plate RP on a rear surface MN opposite to the front (reflective) surface MR of a mirror M that reflects exposure light EL, and to cool the mirror M through radiation (see Japanese Patent Application No. 2002 -222911). FIG. 14 is a schematic structure showing one exemplary cooling method for the mirror M.
There has not been proposed, for example, a temperature adjustment for mitigating a temperature distribution among an illuminated area IE and a non-illuminated area NIE on the mirror M's reflective surface MR, and a rear surface IB of the illuminated area IE, and for maintaining the mirror M at a reference temperature. Therefore, the reflective surface MR of the mirror M thermally deforms due to the temperature distribution inside the mirror M, changes its curvature between the original reflective surface MR and the thermally deformed reflective surface MR′, as shown in FIG. 15, and deteriorating the imaging performance. FIG. 15 is a schematic structure showing a thermally deformed curvature of the mirror M.
In addition, only the mirror's improved internal temperature distribution is insufficient. If the mirror cannot be maintained at the reference temperature, the temperature variance changes a mirror position and deteriorates mirror's optical performance.